This application claims the priority benefit of Taiwan application serial no. 91114481, filed on Jul. 1, 2002.
1. Field of the Invention
This invention relates in general to piezoelectricity and, in particular, to the protection and reliability improvement of piezoelectric workpieces utilizing augmenting surface electrodes.
2. Technical Background
Piezoelectricity is useful in various applications. With the advancements in material science and microelectronic technology, piezoelectric apparatuses are found in ever more equipment, either scientific, industrial, or commercial. Typical examples include piezoelectric transformer in the power system of portable devices such as notebook computers, personal digital assistant (PDA), and cellular phones, accelerometer and gyroscope for commercial navigation devices and piezoelectric signal filter in industrial sensory and control systems, among others.
Workpieces of different physical shapes and sizes of the piezoelectric nature are the center core of piezoelectric systems. Piezoelectric workpieces are made of materials capable of being fabricated to exhibit piezoelectricity. Typically, selected powdered material such as lead zirconate titanate (PZT) is made into the piezoelectric workpiece of the desired shape and size in a sintering-based fabrication procedure. During the high-temperature sintering phase, grains in a molded workpiece are grown. For some commercial piezoelectric material, sintering brings up useful piezoelectricity, but for many others, further processing is necessary. For these materials, a bulk workpiece does not possess piezoelectricity until it is processed in a polarization procedure.
Thermal polarization procedures generally known as poling are employed to orient the electric dipoles in the grain molecules of the workpiece. The aim is so that the grain molecules, in the bulk, exhibit a gross polarization that in general conforms to the desired orientation field pattern of the piezoelectric device for certain designed operating characteristics. To implement the polarization processing on the workpiece under fabrication, electric fields of relatively high voltage are necessary. High voltage needs to be supplied across electrodes adhered (fixedly attached) to the surface of the workpiece for a prolonged period of time so as to facilitate the poling.
One obvious problem with the conventional technique of piezoelectric workpiece fabrication is related to the above-mentioned high-voltage poling. In general, the electrodes used for polarization processing are also the functional electrodes of the same workpiece for its future normal operation. Considering that the polarization voltage is, typically, several to tens of times that of the voltage that will be present across the function electrodes of the workpiece during normal operation, it is likely that the polarization processing during fabrication, if not designed properly, becomes damaging to the workpiece itself. Two possible causes are responsible for such destructive polarization processing results accompanying the relatively-high polarization voltage.
The first is understood to be related to the uneven internal body stress arising from the poling of molecular electric dipoles in the workpiece. As the poling is implemented for an extended period of time, material crystalline grains within the workpiece located between the electrodes supplied with a high electric potential difference are gradually polarized. As the electric dipoles of those grains are gradually polarized, the grains also exhibit an ever more significant piezoelectricity.
Gradually, the workpiece thus experiences partial piezoelectric deformation in the body portion generally between the poling electrodes. Since, as mentioned, this poling voltage is times higher than that of normal operation, such partial structural deformation is likely to create significant internal mechanical stress in the boundary region where the polarized and un-polarized regions meet. Such body stress is sometimes sufficient to break the workpiece into pieces. This is particularly the case if one or more of the electrodes for a piezoelectric workpiece are patterned into shapes with acute angles. Such electrode shape patterns are more likely to induce high regional concentrations of internal mechanical stress within the piezoelectric workpiece.
The second cause is in relation to the phenomenon of point discharge across the electrodes used for polarization processing. This, also, is particularly true if an electrode for a piezoelectric workpiece is patterned into a shape with acute angles to induce point discharge during polarization processing. A point discharge during the poling processing of workpiece fabrication is likely to be catastrophic since the body of the workpiece may be fatally broken apart into pieces. Surge current in association with a point discharge across electrodes of a piezoelectric workpiece inevitably gives rise to abrupt increase in local body mechanical stress along the path of the discharge current. Frequently, as had been observed, such an abrupt regional stress increase breaks up the workpiece into pieces.
After fabrication, operation of a piezoelectric workpiece, as is well known, involves the mechanical/electrical energy conversion. Sustained and reliable operation of a workpiece in a piezoelectric system is dependent on several factors. Among these factors, the bulk physical structural characteristics in the workpiece is an important one.
Although the electric voltages in relation to the operation of a piezoelectric workpiece are relatively much lower than those employed during the fabrication phase for the same workpiece, however, a workpiece is only exposed to fabrication electric fields for the matter of a few, hours. By contrast, reliable operation of a piezoelectric system requires that the workpiece be used for thousands of hours. Thus, stress concentration build-ups within the body of a piezoelectric workpiece not sufficient to fail the device during the fabrication phase may still fail the device during prolonged periods of normal operation. For example, if the piezoelectric device is operated in non-optimized modes, the internal heat build-up is a likely factor to amplify the mechanical stress concentration to a level sufficient to fail the device.
For the foregoing reasons, there is a need for a method to avoid the formation of regions with abrupt polarization orientation alteration inside the body of a piezoelectric workpiece that may lead to mechanical stress concentrations and eventually result in structural failure, either during the manufacturing phase or during normal operation.
There is also a need for a method to smooth the polarization orientation alterations inside the body of a piezoelectric workpiece that prevents the build-up of mechanical stress concentrations to damaging levels, either during the manufacturing phase or during normal operation.
The invention is directed to augmenting surface electrodes for piezoelectric workpieces for improving fabrication and operation reliability thereof. Augmenting surface electrodes for piezoelectric workpieces together with the method for the workpiece fabrication are disclosed. A piezoelectric workpiece used for energy conversion between electrical and mechanical forms in a piezoelectric system comprises a body, a number of function electrodes, and at least an augmenting surface electrode. The body of piezoelectricity is used for implementing the energy conversion. The function electrodes are each fixedly attached to the surface of the body, and are connected in the electric circuit for implementing the energy conversion. At least one of the function electrodes has a shape with a contour of at least one acute angle. At least an augmenting surface electrode has a substantially elongated shape fixedly attached to the surface of the body proximate to the acute angle. Together, the augmenting surface electrode and the proximate function electrode thereof constitute a gross electrode that substantially cancels the acute angle when both are connected electrically to the same electric potential. The acute angle is cancelled during the polarization of electric dipoles of the body grain molecules so that the boundary region between different polarization orientation distribution regions can be smoothed.